Fluorescence light source devices in which green light is emitted from a phosphor as fluorescence by irradiating the phosphor with laser light, which is excitation light, have been conventionally known as green light sources for use in, for example, projectors. A known example of such fluorescence light source devices is a fluorescence light source device including a wavelength conversion member in which the surface of a rotary wheel is coated with a phosphor. The phosphor in the wavelength conversion member generates light in the green region upon irradiation of the wavelength conversion member with laser light in the blue region (see Patent Literature 1).
However, the fluorescence light source device including the wavelength conversion member provided with the rotary wheel has problems in that motor components that rotatively drive the rotary wheel are likely to deteriorate to cause failures and in that the drive system itself has a complex structure.
Another known example of fluorescence light source devices is, as illustrated in FIG. 11, a fluorescence light source device including a wavelength conversion member in which a fluorescence member 41 composed of a YAG sintered body is disposed on the front surface of a substrate 42 with a barium sulfate layer 43 interposed therebetween (see Patent Literature 2). In this fluorescence light source device, the substrate 42 is a Mo—Cu substrate, and a heat-dissipating fin 45 is disposed on the back surface of the substrate 42. In the fluorescence light source device, the fluorescence member 41 generates light in the green region upon irradiation of the fluorescence member 41 with laser light in the blue region, which is excitation light.
However, such a fluorescence light source device has a problem in that excitation light reflects off the front surface of the fluorescence member 41 when the fluorescence member 41 is irradiated with excitation light, which causes an insufficient amount of excitation light to be introduced into the fluorescence member 41 and results in poor luminous efficacy.
There is thus proposed a fluorescence light source device in which an uneven structure is formed on an excitation-light receiving surface of a wavelength conversion member. This uneven structure reduces the reflection of excitation light off the excitation-light receiving surface (see, for example, Patent Literature 3).
Specifically, Patent Literature 3 discloses a fluorescence light source device including a wavelength conversion member in which a light-transmissive substrate having a substantially rectangle plate shape and having an uneven structure with projections arranged on the surface is disposed on a fluorescence member having a rectangle plate shape. The surface of the light-transmissive substrate functions as an excitation-light receiving surface. The fluorescence member is formed by dispersing a phosphor in a sealing material, for example, a glass material such as an inorganic glass or a resin material such as a silicone resin. The light-transmissive substrate is made of a material having a high thermal conductivity, such as sapphire.
However, this fluorescence light source device has been found to have a problem in that high luminous efficacy is not obtained because of an elevated temperature of the wavelength conversion member.
Specifically, the fluorescence member is formed by dispersing a phosphor in a sealing material, such as a silicone resin having a low thermal conductivity (thermal conductivity: 0.1 W/mK), so that the heat generated in the phosphor upon irradiation with excitation light is less likely to be transmitted, which results in an elevated temperature of the phosphor itself. The elevated temperature of the phosphor leads to temperature quenching and results in low conversion efficiency (external quantum efficiency) of converting excitation light into fluorescence, which causes luminance saturation or low luminance and thus generates insufficient fluorescence flux. A heat-dissipating member can be provided only in apart of the back surface of the light-transmissive substrate on which no fluorescence member is disposed, specifically, the peripheral part of the back surface. Therefore, the heat transmitted to the light-transmissive substrate from the fluorescence member needs to be transmitted in the direction perpendicular to the thickness direction of the light-transmissive substrate. In this way, the contact area between the light-transmissive substrate and the heat-dissipating member is small, and the distance over which the heat is transmitted to the heat-dissipating member from the fluorescence member is long, so that sufficient heat dissipation is not achieved.
Furthermore, it has been found that the fluorescence light source device of Patent Literature 3 fails to achieve high luminous efficacy because fluorescence generated inside the wavelength conversion member cannot be used effectively.
Specifically, part of fluorescence emitted from the phosphor inside the wavelength conversion member and entering the light-transmissive substrate does not exit from the front surface of the light-transmissive substrate or the fluorescence-exiting surface composed of the front surface and the side surface of the fluorescence member. The part of fluorescence repeatedly reflects in the light-transmissive substrate and exits from the side surface of the light-transmissive substrate to the outside. Since the fluorescence generated inside the wavelength conversion member cannot exit from the fluorescence-exiting surface at high efficiency in this way, fluorescence cannot be used effectively. In addition, the length and width dimensions of the light-transmissive substrate are larger than the length and width dimensions of the fluorescence member. The size of a light source of fluorescence is thus set in accordance with the length and width dimensions of the light-transmissive substrate. Therefore, the size of a light source of fluorescence is excessively large in regard to the relationship with the length and width dimensions of the fluorescence member, and the fluorescence emitted from the wavelength conversion member is guided by using an optical member such as a reflecting mirror, which leads to limited etendue. As a result, the fluorescence emitted from the wavelength conversion member cannot be used effectively.